Inverters

schmekschmek Posts: 34Registered Users ✭✭
Hi,

I am a bit confused about the application for different inverters. I've read about string inverters, central inverters and microgrid inverters being the most common inverters for solar systems. But how does these solar inverters act in relation with a battery? Can you use one string inverter for both the PV array and the battery pack? Or do you always want to use a hybrid inverter when a battery pack is involved?

Kind regards,
Sindre 

Comments

  • littleharbor2littleharbor2 Posts: 979Solar Expert ✭✭✭✭
    The inverters mentioned are grid tie inverters. The grid is the "Battery"  "Off Grid" battery based inverters are for stand alone systems and run off stored energy in your battery bank. Hybrid inverters are a combination of both. Are considerably more complex and expensive. They can be configured to never import to the grid in the case you don't want to have an agreement with the POCO.  You  run off solar power  till you need the grid, then seamlessly transition to grid power. You can have a minimal battery bank which only comes into play in the event of a grid power outage.  there are many intermediate configurations available as well which can be beneficial with different TOU (time of use) power company plans.

     If you have the money and don't mind the expense and hassle of maintaining and replacing batteries then a Hybrid set up can work. Just keep in mind that whenever you add batteries into a PV system you'll never save money over a straight grid tie system. If you have an unreliable grid  with occasional long outages then Hybrid might be the best way to go.  A whole house backup generator can be a whole lot cheaper and easier to maintain.  

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mcgivormcgivor Posts: 2,046Solar Expert ✭✭✭✭
    String, central and micro inverters are grid interactive devices, when a battery is involved, a charge controller is required. There are inverters which are battery based grid interactive with built in chargers which can be used with or without solar which is what you would require with a battery involved, but a charge controller would be essential with a battery and solar combination. With a clear description of what it is you're hoping to achieve would allow responses to be more precise, unless you're just curious.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • schmekschmek Posts: 34Registered Users ✭✭
    Hi,

    Thank you for your answers.

    This is an off grid energy system, so it will not interact with the grid at any time. I'm aware of the need of a charge controller. The system will include a solar array, battery pack, and diesel generators. My concern lies in whether this makes stringinverters and microinverters non applicable? In other words, I need an inverter to connect to both my battery pack and solar array. I am assuming this will be more efficiently with one hybrid inverter, compared to two seperate inverters? 

    In addition, I want the possibility of charging the batteries with AC generators, so I suppose a bi-directional hybrid inverter is the best option? I am basically wondering if this system can rely on one component for AC-DC, DC-AC conversion, or if a more advanced system is necassary. 

    Sindre
  • littleharbor2littleharbor2 Posts: 979Solar Expert ✭✭✭✭
    edited April 23 #5
    Most, if not all high end off grid inverters have generator charging function built in. you wouldn't need another , separate inverter.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mcgivormcgivor Posts: 2,046Solar Expert ✭✭✭✭
    The string and micro inverters will not work for the application described, an inverter charger with an AC input would allow a generator to supplement the needs of the batteries if required, a hybrid would involve  the grid, so not a requirement,  even though an inverter charger can use a grid feed, a generator would be the alternative source to supplement charging if required.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    Generally speaking, for an off-grid application, you'll want to stay with gear designed for off-grid. Many of the micro-inverter etc products rely on a grid frequency to operate, which obviously isn't present in an off-grid application.

    The off-grid inverter doesn't interact directly with pv, though it may be networked to a charge controller which does.

    As LH2 notes, except for small systems, most off -grid inverters include AC chargers, and buying this type tends to be cheaper than buying a good quality separate charger.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • schmekschmek Posts: 34Registered Users ✭✭
    Thanks a lot for your answers! :)
  • BB.BB. Posts: 27,682Super Moderators admin
    edited April 24 #9
    It is possible to use Grid Tied Central or Micro Inverters with a pure off grid AC inverter system.

    More or less, the Off Grid AC inverter is a bi-directional power device... Which are used with the Off Grid (usually a true sine wave AC) inverter supplying power to the AC output/micro grid. However, if you try to feed syncronized AC power into the micro grid, it can push energy backwards through the off grid AC inverter (power from the GT Inverters) and recharge the battery bank.

    In times past, unless you put a dump load or some other system in place, the battery bank would over charge from the "unregulated" GT solar back feed.

    Today, there are off grid AC inverters (battery based) that vary the line frequency by 60 Hz +/- 1 Hz or so when the battery bank is full. This "knocks the GT inverters off line" for ~5 minutes and/or until the line frequency is back at 60.0 Hz. And the GT inverters start feeding AC power back into the micro grid (to be shared with AC loads and AC off grid inverter).

    Does it work. Yes. Is it ideal (from cost, maintenance, etc.) point of view vs just a solar array and standard Solar Charge Controller... less than clear.

    SMA makes the "Sunny Island" system which does better than knocking the GT inverters off grid, they vary the AC micro frequency to "throttle" the GT inverters between 0 and 100% output. That is a better charging scheme... And can be easier to integrate in a "village" sized micro-grid system. Is there an advantage to this system cost/design wise--Have to design several paper systems and see what works out best for your needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Posts: 3,884Solar Expert ✭✭✭✭
    Agree Bill but I would say it makes more sense to AC couple a very very large system (a micro-grid) with grid based inverters to keep a battery based inverter power system charged and running.

    Outback and Schneider (over 100KW and larger) along with SMA have been doing this for quite some time. It is not a do it yourself project but does also allow someone to add a battery back-up system for one of the various reasons a grid person would want this.

    For the typical grid tie people on this forum, it makes much more sense if you want a battery system to just change the systems grid inverter to an Outback Radian or an XW from Schneider or the SMA sunny.

    For someone offgrid it makes much more sense to DC couple and avoid the complexity of AC coupling. Not to mention AC coupling takes a toll on reliability that DC coupling avoids. I just finished a project where I had to tell the company there are only 2 things that should shut down AC power offgrid. They are a dead battery and blown or tripped over current protection. I guess acts of nature/man might cause a problem that I do not want to contemplate.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • clockmanfranclockmanfran Posts: 22Registered Users ✭✭
    edited April 23 #11

    As this is in the solar beginners section........

    Bill, I am a newbie here so won't say to much, but I disagree with The Sunny Island and its HZ increase to knock off the SB GTI's.

    The way the concept is portrayed by SMA it sounds great. However in reality the HZ with the SI is unstable and climbs and max's out at 60HZ for a 50HZ system, at 60HZ domestic appliances fail.

    And as SMA said later, you need lots of their expensive interface equipment to get all of their machines to talk to each other, which, after getting a costly Sunny Island is info you do not want to here.

    SMA have a gagging order on me so that's all I can say, but as you probably realised with John, 'Oztules' help we developed the OzInverter.

    The OzInverter is simple, robust and very cost effective, approximately $600, even cheaper if you scavenge stuff.       It does a true 6kW all day if you want, and can handle AC coupling up to about 12kW,  and as its a H bridge design so can back charge the batteries. It can also handle DC coupling to the same 48vdc batteries at the same time as AC coupling without throwing any Wobblies like the SI does.

    I just do the Power, control & cooling PCB boards and a booklet HOW TO MAKE A 6kW INVERTER. Its not a business, but may get one of my teenage boys putting together a basic kit of parts one day.

    AC coupling is the way forward, as we use the GTI's output to feed the mini grid the OzInverter has created, so in essence the batteries only get called upon for a few minutes as things wake up.

    We knock off each GTI with voltage comparator simple programmable circuit DVB01, that monitor the DC battery voltage and have time hysteresis and arranged to shut down other GTI's sequentially. 

    The GTI's I use are good ones but second hand/used and very cheap, but tend to be LF (toroid inside), although some folk out there are running with HF type GTI's but there are issues with HF spikes now and then so a little more filtering is required.

    The OzInverter is running and being put together by people all over the World, and gives small communities self empowerment and cost effective electricity.

    I have not put any utub links or other links with this post.



     


    Everything is possible, just give me Time.

    The OzInverter man. Normandy France.

    3off Hugh P's 3.7m dia wind turbines, (9 years running).  ... 5kW PV on 3 Trackers, (5 years) .... 9kW PV AC coupled using Used/second hand GTI's, on my OzInverter created Grid, and back charging with the AC Coupling and OzInverter to my 48v 1300ah batteries. 

  • schmekschmek Posts: 34Registered Users ✭✭
    On another note, when manufactorers of battery packs sells battery packs for example to use on a marine application (ship). They will implement a conversion device AC-DC and DC-AC in the storage system right?
  • Dave AngeliniDave Angelini Posts: 3,884Solar Expert ✭✭✭✭
    In marine applications, most have an Inverter/charger that does all conversion and inversion. It has a transfer switch built in to work with shore power or a genset.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • BB.BB. Posts: 27,682Super Moderators admin
    Converters are used when needed... I don't see anything different with marine systems...

    In general, higher voltage AC systems tend to be safer and more reliable than higher voltage DC systems.

    Also, battery banks tend to have a rather wide voltage range (12 volt lead acid, 10.5 to 16+ volts; 3.7 volt Li Ion somewhere between 3.0 and 4.2 volts). For AC systems, roughly 117 to 130 VAC (US/North American).

    Wide voltage ranges tend to need something to "stabilize" the voltage range to something more useful... Whether that is a DC to AC inverter, or a DC to DC converter, just depends on the needs at hand.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    The inverter on my boat is very similar to the one in my off-grid cabin. Minor differences include a slightly different transfer switch that takes into account a neutral ground bond in shorepower connection.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • schmekschmek Posts: 34Registered Users ✭✭
    edited April 24 #16
    Okay I see. So this means the battery pack will include a inverter/charger to connect with the diesel generators. But what about the solar system, does PV power also go into the same converter system? 

    An important thing is also that i want to integrate the battery and solar array to an already existing diesel generator system. 
  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    The dc pv current goes through a charge controller to the battery bank. Dc current goes from the bank to the inverter, which inverts the dc current to ac for ac loads. Power from pv will charge batteries and/or power loads (including ac loads, via the inverter) depending on battery state of charge and controller settings.

    If the (ac) generator is running, a transfer switch in the inverter is activated, passing ac current to loads and if necessary rectifying some of the generator ac current to dc to charge batteries.

    The inverter and charge controller are generally separate components wired to the bank. With proper inverter, charge controller, and batteries, adding a solar system to a generator is quite common. That's what I did (added pv system to a 4kw diesel) at my cabin.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • schmekschmek Posts: 34Registered Users ✭✭
    For a 1000 kWh/day and peak load of 110 kW, would it be most convenient to AC couple, rather than DC couple? I've read about for example schneider's solutions for off grid systems with AC coupling.


  • Dave AngeliniDave Angelini Posts: 3,884Solar Expert ✭✭✭✭
    Convenience, efficiency, and reliability are design parameters that are chosen for a specific case/project.

     In a very large micro-grid AC vs DC coupling could come down to is the solar or charge source all in one location or spread out over many locations?

    What is it you are planning? I can point you to an Engineer at Schneider who can assist you.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • schmekschmek Posts: 34Registered Users ✭✭
    I would appreciate of you can point me to an engineer at Schneider :smile: 

    There will be one floating solar platform with a capacity of around 180 kW, and a battery bank with capacity of around 120 kWh. The rest of the energy needs will be supplied by diesel generators of 80 kW. Note that the floating solar platform is about 100 meters from the barge, where the rest of the components are located. The system is created for a fish farm in Norway.

    Kind regards
  • Dave AngeliniDave Angelini Posts: 3,884Solar Expert ✭✭✭✭
    Drop me an e-mail at my address below. Can I give that e-mail to an engineer at Schneider?
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • schmekschmek Posts: 34Registered Users ✭✭
    Hi, I've sent you an Email.

    thanks a lot!
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